1. Field of the Invention
The present invention relates to a microelectronic device equipped with a thermal protection circuit and to a thermal protection method for a microelectronic device.
2. Discussion of the Related Art
As is known, several microelectronic circuits and devices require thermal protection in order to prevent damage caused by overheating. For example, thermal protection circuits are normally integrated in power amplifiers and voltage sources or regulators, because operation of power components may easily cause dangerous self-heating.
According to a widely used solution, the power component causing overheating is switched on and off, based on the response of a temperature sensing element, that is preferably integrated in the same die as the power component. The temperature sensing element normally comprises a forward-biased junction component, the on-state current whereof depends on temperature.
For the sake of clarity, reference will be made to FIG. 1, that shows a power MOS transistor 1 driven by an amplifier 2 and provided with a thermal protection circuit 3.
The thermal protection circuit 3 comprises a temperature sensing element, here a PN-junction diode 5, a current source 6, for supplying the temperature sensing element 5 with a sense current IS, a first protection transistor 7, supplied by a further current source 8, a threshold circuit 9 and a second protection transistor 10.
The diode 5 is forward biased and a sense voltage VS on its anode 5a is nearly linearly dependent on temperature (approximately −2 mV/K). The first protection transistor 7 has its gate and drain terminals respectively connected to the anode 5a of the diode 5 and to an input of the threshold circuit 9, that is herein an inverter and has a switch threshold. The second protection transistor 10, here of the PMOS type, has its source and drain terminals respectively connected to a voltage supply line 11 and to a gate terminal of the power transistor 1, and is coupled to the threshold circuit 9 for receiving a control signal VC on its gate terminal.
In normal operation, the sense voltage VS on the anode 5a of the diode 5 is high so as to keep the first protection transistor 7 in the on-state. As long as the voltage on the input of the threshold circuit 9 is below the switch threshold, the control signal VC has an enable value (high in the example) for keeping the second protection transistor 10 in the off-state and enabling operation of the power transistor 1. As temperature increases, the sense voltage VS decreases until the first protection circuit 7 is turned off. The voltage on the input of the threshold circuit 9 then jumps above the switch threshold and causes the control voltage VS to switch to a shutdown value (low), for turning the second protection transistor 10 on. The power transistor 1 is thus cut off, because its gate terminal is stuck at a supply voltage VCC provided by the voltage supply line 11. Accordingly, acceptable temperature conditions may be restored and overheating is prevented.
Known thermal protection circuits, however, suffer from some drawbacks. In the first place, quite large additional currents are required, because the thermal sensing elements are based on forward-biased junction elements. The total quiescent current of the integrated device is therefore significantly increased, which is always undesirable and is especially disadvantageous in battery-operated apparatus. Solutions for reducing current consumption are sometimes available, but they normally require additional components, such as resistors or bipolar transistors. Therefore, the die area necessary to accommodate thermal protection circuits is increased, in spite of the fact that current reduction is only partially achieved and may not be satisfactory.